The present invention relates to fire extinguishing and smoke producing methods and associated systems and, more particularly, to methods and related systems which do not involve halocarbons and which are highly effective in extinguishing fires and/or in setting up smoke screens, even when relatively small quantities of chemicals are used yet are nontoxic.
The present invention relates, in particular, to methods and systems for volume fire extinguishing, some of which methods can also be used to create an effective and nontoxic smoke screen. Throughout most of the subsequent discussion, reference will be made largely to fire extinguishing applications of methods and systems according to the present invention, with only brief mention of use of such methods and systems in the creation of smoke. It is intended that both applications, as well as others, fall within the scope of the present invention.
Volume fire extinguishing involves the temporary creation of an atmosphere which is incapable of sustaining combustion within the volume to be protected, typically a relatively confined volume, or by applying a stream of extinguishing agent to the base of the flame which is known as local application and is commonly practiced using portable fire extinguishers.
One of the volume fire extinguishing methods in most widespread use at present includes the introduction of volatile halocarbons, such as Halon 1301, for example, into the volume to be protected. One of the extinguishing agents which are presently commonly used for location applications is Halon 1211. Halocarbons have excellent fire extinguishing capacity which is attributable to their being inhibitors of combustion. Halocarbons actively interfere with the chemical reactions taking place in the flame and effectively inhibit them.
Furthermore, halocarbons have a number of desirable properties such as low toxicity. In addition, halocarbons gases can be rather easily liquefied under pressure, making them easily storable in the liquefied state. Halocarbons do not adversely affect equipment and other materials with which they come in contact.
Nevertheless, halocarbons suffer from a fundamental disadvantage, namely, they are known to interact with ozone, which leads to the destruction of the earth's ozone layer. According to the 1987 Montreal Protocol, which prescribed a number of international measures for the protection of the earth's ozone layer, the use of halocarbons is to be completely banned by the year 2000.
Another commonly used total flooding and local application extinguishing agent is CO.sub.2. Because of its high weight-to-extinguishing-power ratio and other health considerations, the use of carbon dioxide has been drastically reduced as halons have gained wider acceptance.
It is thus quite urgent to find alternative volume fire extinguishing means which could successfully act as a replacement for halocarbons or to enhance the performance of other commonly used extinguishing agents, such as CO.sub.2, and the like. A successful replacement for halocarbons would possess a volume and local fire extinguishing effectiveness at least equal to that of halocarbons, yet would be ecologically safe and nontoxic.
Two basic types of such ecologically benign fire extinguishing materials are presently known. The first includes inert gaseous diluents, such as carbon dioxide, nitrogen, water vapor, and the like. The second type includes fire extinguishing powders based on mineral salts, such as carbonates, bicarbonates, alkali metal chlorides, ammonium phosphates, and the like.
As presently implemented, both types of materials suffer from serious disadvantages. Inert gaseous diluents are largely ineffective in disrupting the reactions taking place in the flame. Rather, inert diluents act by diluting the air in the volume being protected, thereby lowering the oxygen concentration below that required to sustain the combustion. An example of the use of inert diluents is disclosed in U.S. Pat. No. 4,601,344 to Reed. Reed relates to a gas generating composition containing glycidyl azide polymer and a high nitrogen content additive which generates large quantities of nitrogen gas upon burning and can be used to extinguish fires.
For relatively airtight volumes, the amount of diluent required roughly equals the amount of air already in the volume prior to combustion. If the volume to be protected is not airtight, the required volume of the inert diluent must be several times that of the protected volume.
Fire extinguishing methods based on inert dilution require relatively large amounts of diluent and are appreciably less effective and reliable than extinguishing with halocarbons.
Volume fire extinguishing with the help of powders is carried out by dispensing a powder aerosol in the volume to be protected. The aerosol envelops the flame thereby suppressing it. It is believed that powders chemically interrupt combustion by forcing the recombination and deactivation of chain propagators responsible for sustaining the combustion process in the focus of fire.
Such recombination is believed to occur both at the surface of the solid particles of the aerosol and, to some extent, also in reactions of the chain propagators with gaseous products of the evaporation and decomposition of powders in the flame. Chain propagators are gaseous atomic particles or radicals having a free valence, which serve to initiate and sustain the branched chain reactions characteristic of combustion processes in combustible substances containing carbon.
However, the efficiency of presently implemented volume fire extinguishing with the help of powders is also of limited efficacy because of the comparatively low dispersity of the fire-extinguishing powders. The particle size of presently used powders ranges from about 20 to about 60 microns. Such large particles have a relatively low surface-to-volume ratio. Since the desired reactions take place largely on the surface of the particles, a given amount of such powders has a limited capacity for interrupting the chain reactions and putting out the fire.
Further, it is difficult to prepare an aerosol of such powders which will distribute uniformly throughout the volume to be protected. It is, in addition, difficult to ensure that the powder particles, once formed, will stay in their original suspended state while stored for a sufficiently long period prior to use so as to maintain the viability of the product as a fire extinguishing composition. Finely-dispersed powders have a strong tendency to agglomerate, or cake, during storage. Such agglomeration greatly hinders the dispensing of the material from its storage container during use. Furthermore, whatever particles are able to leave the storage container and come in contact with the fire are relatively coarse-grained powder particles, having a relatively low surface-area-to-volume ratio and thus possessing reduced fire extinguishing capacity per unit weight.
Attempts have been made to solve the problems associated with the long-term storage of finely divided powders. Exemplary of such attempts is U.S. Pat. No. 4,234,432 to Tarpley, which discloses a powder dissemination composition in which the powder is contained in a thixotropic gel which prevents the agglomeration, sintering, and packing of the powder material. The finely divided powder has at least a bimodal particle distribution size distribution encapsulated in a gelled liquid. The method appears to be complex, requiring the fabrication of a powder of a well-defined particle size distribution.
In at least one case, attempts have been made to get around the storage problems by storing reaction precursors rather than the actual powders. U.S. Statutory Invention Registration No. H349 to Krevitz et al. discloses reagent compositions which are chemically inert when solid and are chemically active when molten. The reagent compositions may comprise a first substance such as a high molecular weight wax or polymer and a second substance which is dissolved, dispersed, or encapsulated in a solid matrix of the first substance. The second substance is a highly chemically reactive compound such as a strong base or a strong acid. As solids, the reagent compositions are inert. When molten, the second substance is exposed and the resultant liquid solutions are highly reactive.
There is thus a widely recognized need for fire extinguishing methods and systems which are at least as effective as those involving the use of halocarbons but which are ecologically safe.
Specifically, there is a clear need for, and it would be highly advantageous and desirable to have, fire extinguishing methods and systems which use chemicals which do not adversely affect the earth's ozone layer and which are capable of putting out fires quickly and efficiently.
In addition, there is a widely recognized need for smoke creating methods and systems which are highly effective yet are not toxic.